VISUAL WORK WITH THE MICROSCOPE. 489 



available, it fulfils every requirement. The lamp I use is one of 

 those with a metal chimney, and a piece of glass in front, and 

 with a Nelson lens on a swinging arm, on which it can be 

 adjusted. 



For critical work, the edge of the flame is best, as it gives a, 

 far brighter illumination than the flat, and therefore stands 

 better the addition of screens. With the flat of the flame, how- 

 ever, one can obtain excellent results with low powders, and it is 

 rather easier to adjust. 



Using the edge of the flame, which I do myself with all 

 powers, it is sometimes difficult to obtain, especially with low 

 powers, absolutely equal illumination all over the field. To 

 remedy this, I have made a slight modification of my lamp, 

 which, if it has not entirely removed the defect, has certainly 

 produced a great improvement. It consists in fitting to the 

 inside of the lamp, quite close in front of the flame, and in fact 

 in a line with its front edge, a thin metal plate, with an oblong 

 opening, 3/8ths of an inch in width, and just a little less than the 

 length of the flame. It cuts ofi;* the light coming from the 

 posterior half of the flame which is usually taken up by 

 the outer part of the collecting lens, and certainly gives very 

 much more uniform illumination of the field, even with low 

 powers. 



The Nernst lamp is excellent for occasional and unusually 

 difficult work, superior, I think, to the kerosene lamp, but it 

 requires to be used with dense colour screens, and even then I 

 find it too intense for continuous visual work. For dark-ground 

 illumination it is perfect. 



2. Collecting Lenses. 



Although many workers say that for critical work they find 

 the lamp flame, without the interposition of any collecting lens, 

 best, personally I prefer the collecting lens, which becomes the 

 source of light. This is placed near the lamp and focused so as 

 to throw a nearly sharp enlarged image of the flame on the sub- 

 stage mirror ; and the substage condenser, if then focused so as 

 to produce critical illumination, gives us a sharp image of the 

 partially closed collecting-lens iris on the field. If no collecting 

 lens is used we have the field only partially illuminated, with 

 either a bright streak or other form depending on the source of 



